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Subject: RE: [xdi] Unifying the 3 XDI Markup Models

Nat, sorry, I never got a chance to reply when you orginally sent this. I
just wanted to say that both of your points below - in fact all semantics of
the XDI schema(s) - are a next key focus of our work. Dave and I took the
action items from yesterday's call to write up analyses of the two schema
proposals (the Envelope proposal, which he's going to upload) and the
unified proposal, which covers all 3 markeup models.

So I look forward to a particularly lively discussion of all of this on next
week's call.



-----Original Message-----
From: Sakimura, Nat [mailto:n-sakimura@nri.co.jp] 
Sent: Wednesday, November 24, 2004 4:03 AM
To: Drummond Reed; xdi@lists.oasis-open.org
Subject: RE: [xdi] Unifying the 3 XDI Markup Models

Thanks Drummond. 

Since I may not be available at this phone call (the time is 
from mid-night to 2am) so, I am commenting here. 

(1) The model which is presented here is actually very close to 
the one that I presented at New Orleans f2f, apart from the 
choice of tag names. I am naturally amiable to the schema, 
but I do not like the tag names, especially <instance>. 
This is terribly misleading to the object-oriented people, IMHO. 
At least, it took me a long time to grasp the intent. If we could 
devise a better tag name, that is better I think. 

(2) I am kind of vague about the use of ref and xref. I would 
appreciate if you could explain it. Actually, most people 
would require the difference of XRI/XRef/Ref, I think. 

Wow, I have to get out of the building now ( it is being closed.. 
this is not my office.) 

I will continue later. 



> -----Original Message-----
> From: Drummond Reed [mailto:drummond.reed@cordance.net] 
> Sent: Wednesday, November 24, 2004 5:48 PM
> To: xdi@lists.oasis-open.org
> Subject: [xdi] Unifying the 3 XDI Markup Models
> XDI TC Members and Observers,
> To continue the excellent progress we had been making since 
> the Denver f2f on reconciling the differences between the 
> different schema proposals, I met in person last Thursday 
> with Dave McAlpin and Loren West. We had a highly productive 
> session at which we reached what we all felt was a big 
> breakthrough in understanding the tension between these 
> different approaches.
> We finally realized the conflict was rooted in different 
> underlying assumptions about the XDI markup model. The model 
> Dave has been advocating is "XML-centric", i.e., it starts 
> from the assumption that the data is already marked up in XML 
> (or will be marked up in XML), so the goal is to add XDI 
> markup while still reusing all the existing XML markup.
> The model I have been advocating is "XDI-centric", i.e., it 
> starts from the assumption that the raw data is going to 
> marked up directly into XDI format, and that data already 
> marked up into XML format is the exception rather than the rule.
> We realized, of course, that both models are completely 
> valid. Calling either one "right" or "wrong" would be as 
> silly as calling Cartesian or radian coordinate systems 
> "right" or "wrong". They are simply two different models for 
> serializing the XDI graph - one model optimized for reuse of 
> data and metadata in existing XML graphs, and one optimized 
> for new XDI graphs.
> Obviously for XDI to be successful, both models should be 
> supported, i.e., an XDI processor should be able to 
> understand and parse both.
> We further realized that the XML-centric model has two forms, 
> as illustrated by Dave's examples over the past two calls. 
> One is when the native schema is extensible, in which case 
> the XDI markup can be placed directly inline with the rest of 
> the XML elements (and every XDI element by definition 
> describes its parent element). The other is when the schema 
> for the native XML document is not extensible, so the XML 
> needs to be "wrapped" in an XDI envelope. 
> For easy reference, we labelled these three different XDI 
> markup models as
> follows:
> 1) "Inline" or "Direct" is when the XDI markup is placed 
> directly into an XML instance document whose schema(s) are 
> extensible. The root element of this XML document does not 
> change. This mode will typically be preferred for XDI 
> adaptation/enhancement of XML-centric applications that have 
> been designed or can be easily modified to use extensible XML schemas.
> 2) "Enveloped" or "Wrapped" is when the XDI markup is wrapped 
> around a single XML instance document because the native XML 
> schema is not directly extensible. In this case the root 
> element of the outer wrapper is from the XDI schema 
> (specifically, the "XDI-XML" element in the v1 unified schema 
> proposal), but the native XML document inside the wrapper 
> stays intact. This mode will be typically be preferred for 
> XDI adaptation/enhancement of XML-centric applications that 
> do not use extensible schemas.
> 3) "Open" or "Nested" applies to an XDI document for which 
> the root element is the XDI Resource element, and which 
> therefore allows any number of nested Resources. This mode 
> will be typically be preferred for XDI dictionaries, 
> XDI-centric applications, or even large distributed XML 
> applications whose primary challenge is needing to share data 
> across a large number of different XML schemas.
> In cooking on this after our session, it became apparent that 
> it would be a tremendous virtue to make all 3 models 
> interoperable. In other words, the Open model should be able 
> to contain instances of either the Enveloped or Inline 
> models, and the Enveloped model should be able to contain an 
> instance of the Inline model (for cases where parts of an XML 
> schema are extensible and parts are not.)
> So it became very attractive to develop a single unified XDI 
> schema that supports all three models. This turns out to be 
> relatively easy because the majority of the schema describes 
> the Open model (since that's where XDI itself describes the 
> document structure), while a single element, "XDI-XML", 
> provides the "switchover point" from the XDI-centric model to 
> the XML-centric model.
> Once inside the XDI-XML element, the only structure needed is 
> an "XMLMeta"
> element and an "XMLData" element to support the Enveloped 
> model. This is because the Inline model doesn't need any 
> other elements - it will just reuse elements from the XDI 
> schema or from other extensions written for the "XMLMeta" 
> element (see below).
> Lastly, I added an "XRI-XML" element to support basic XDI 
> addressing inside either the XMLMeta element for the 
> Enveloped model or anywhere in the Inline model. The full The 
> v1 unified schema proposal has been uploaded to:
> http://www.oasis-open.org/committees/download.php/10237/draft-
> xdi-unified-sc
> hema-v1.xsd. 
> Another beauty of this approach is that it provides two clear 
> paths for extending XDI vocabulary - one XML-centric and one 
> XDI-centric:
> 1) The XML-centric option is to write a new XML schema 
> defining a new element for use inside the XMLMeta element, 
> per the SOAP model we have been discussing for some time.
> 2) The XDI-centric option is to add a new XDI resource to an 
> XDI dictionary, then use it in the Open model to describe 
> other resources.
> Both would be equally valid ways of extending XDI, depending 
> on your requirements, and both would be understood by XDI processors.
> Obviously this will be the main topic of our call tomorrow. 
> To help ground the discussion, I have uploaded three example 
> documents:
> 1) A third version of a short-form XDI business card using 
> the v1 unified schema (very few changes from the previous version):
> http://www.oasis-open.org/committees/download.php/10238/draft-
> example-bizcar
> d-short-v3.xml
> 2) A third version of the long-form (including all 
> references) XDI business card also using the v1 unified 
> schema (again, very few changes from the previous version):
> http://www.oasis-open.org/committees/download.php/10239/draft-
> example-bizcar
> d-long-v3.xml
> 3) Finally, a second version of Dave's original XML document 
> markup example, now using the v1 unified schema. Note that 
> this is just an adaptation of his original example, so he, 
> Loren, and others may have many different ideas of how 
> XDI-XML should work:
> http://www.oasis-open.org/committees/download.php/10240/draft-
> example-XDI-XM
> L-v2.xml 
> =Drummond

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